In the case of a one-dimensional structure, a diffraction grating is utilized in optical systems of various devices, as a spectral element having a periodic structure composed of a large number of parallel members, and, in recent years, its application to X-ray imaging devices has also been attempted. In terms of a diffraction process, the diffraction grating can be classified into a transmissive diffraction grating and a reflective diffraction grating, wherein the transmissive diffraction grating includes an amplitude-type diffraction grating (absorptive diffraction grating) in which a plurality of light-absorbing (absorptive) members are periodically arranged on a light-transmissive substrate, and a phase-type diffraction grating in which a plurality of optical phase-shifting members are periodically arranged on a light-transmissive substrate. As used herein, the term “absorption (absorptive)” means that light is absorbed by a diffraction grating at a rate of greater than 50%, and the term “transmission (transmissive)” means that light is transmitted through a diffraction grating at a rate of greater than 50%.
A diffraction grating for near infrared light, visible light, or ultraviolet light can be relatively easily produced, because near infrared light, visible light and ultraviolet light are sufficiently absorbed by a very thin metal. For example, an amplitude-type diffraction grating based on a metal grating structure is produced by vapor-depositing a metal on a substrate made of glass or the like to form a metal film on the substrate and patterning the metal film to form a grating structure. In an amplitude-type diffraction grating for visible light, when aluminum (Al) is used as the metal, it is enough for the metal film to have a thickness, for example, of about 100 nm, because a transmittance of aluminum with respect to visible light (about 400 nm to about 800 nm) is 0.001% or less.
On the other hand, as is well known, X-ray is very low in terms of absorption by a material, and is not so large in terms of phase shift, in general. Even in the case where a diffraction grating for X-ray is produced using gold (Au) as a relatively favorable material, a required thickness of gold is about several ten μm. As above, in an X-ray diffraction grating, when a periodic structure is formed by arranging a transmissive member and an absorptive member or phase-shifting member which are even in width, at a pitch of several μm to several ten μm, a ratio of thickness to width (aspect ratio=thickness/width) in the gold portion has a high value of 5 or more
Silicon fabrication techniques are suitable for forming such a periodic structure having a high aspect ratio, and a production method for such a metal grating structure is disclosed, for example, in the following Patent Literatures 1 and 2. A metal grating structure production method disclosed in the Patent Literatures 1 and 2 include: a resist layer forming step of forming a resist layer on a principal surface of a silicon substrate; a patterning step of patterning the resist layer and removing the patterned portion of the resist layer; an etching step of etching a portion of the silicon substrate corresponding to the removed portion of the resist layer by dry etching to thereby form a recess having a given depth; an insulation layer forming step of forming an insulation layer on an inner surface of the recess of the silicon substrate; a removal step of removing a portion of the insulation layer formed on a bottom of the recess; and an electroforming step of applying voltage across the silicon substrate to perform an electroforming process to thereby fill the recess with a metal, wherein an anodic oxidation process or a thermal oxidation process is used in the insulation layer forming step.
Meanwhile, in the case where a silicon oxide film (silicon dioxide (quartz, SiO2) film (layer)) is used as the above insulation film when an X-ray metal grating structure is produced by the metal grating structure production method disclosed in the Patent Literatures 1 and 2, a thermal expansion coefficient of silicon dioxide is about 0.7×10−6/K, whereas a thermal expansion coefficient of silicon is about 2.6×10−6/K. For this reason, when a silicon oxide film serving as the above insulation layer is formed on the silicon substrate at a high temperature by a thermal oxidization process, and subsequently the silicon substrate formed with the silicon oxide film is cooled to normal temperature, due to a difference in thermal expansion coefficient between silicon and silicon dioxide, a thermal stress is generated in an X-ray metal grating structure produced from the silicon substrate. Therefore, in a process of producing an X-ray metal grating structure from a silicon substrate (silicon wafer), this thermal stress causes a strain in the X-ray metal grating structure, and thereby flatness of the X-ray metal grating structure deteriorates compared to flatness of the silicon substrate (silicon wafer).
Further, in the metal grating structure production method disclosed in the Patent Literatures 1 and 2, the metal grows from the bottom of the recess by an electroforming process (bottom-up growth). The present inventor found a phenomenon that, during this growth, the metal grows in such a manner that a width of a top thereof (a region adjacent to an opening of the recess) becomes slightly larger than a width of a bottom thereof. Then, due to this slight difference in width between the bottom and the top of the metal, a stress (hereinafter referred to appropriately as “electroforming stress (plating stress)” is generated. This electroforming stress also causes a strain in the X-ray metal grating structure, and thereby the flatness of the X-ray metal grating structure deteriorates compared to the flatness of the silicon substrate (silicon wafer).